users: Add support for changing user role to moderator in api.

This commit adds backend support for changing a user role to
moderator and also to change role from moderator to any other
role.

zerver/models.py

@@ -1186,6 +1186,7 @@ class UserProfile(AbstractBaseUser, PermissionsMixin):
     ROLE_TYPES = [
         ROLE_REALM_OWNER,
         ROLE_REALM_ADMINISTRATOR,
+        ROLE_MODERATOR,
         ROLE_MEMBER,
         ROLE_GUEST,
     ]